1. Field of the Invention
This invention relates to a pin-photodiode and an avalanche photodiode for sensing near-ultraviolet to blue rays. A photodetector is a sensor which receives light and generates a photocurrent output which is in proportion to the power of input light. A photodiode and a phototransistor having pn-junctions are photodetectors as solid state devices. A photoconductive device without a pn-junction senses the light power by photoconductive effects. Besides the semiconductor devices, there are a phototube or a photomultiplier photodetectors which make use of vacuum tubes.
The present invention relates to a near-ultraviolet-blue photodiode having a pn-junction.
This application claims the priority of Japanese Patent Applications No. 2001-270031 filed on Sep. 6, 2001 and No. 2001-324341 filed on Oct. 23, 2001, which are incorporated herein by reference.
Several kinds of photodiodes have been made and practically used for detecting light of various wavelength ranges. A purpose of the present invention is to provide a photodiode for sensing near-ultraviolet to blue rays of a wavelength between 460 nm and 300 nm. A more concrete purpose is to provide a photodiode for detecting light of a wavelength of 400 nm of HD-DVDs (high definition digital video disc) which will be manufactured and will be sold on the market in the near future.
This invention relates also to a blue-ultraviolet avalanche photodiode (APD) for sensing blue, violet and ultraviolet rays of a wavelength range between about 460 nm and 300 nm. An avalanche photodiode is a high-sensitive photodetector of avalanche-amplifying photocarriers by applying a high reverse bias slightly below a breakdown voltage upon a pn-junction. There has been no blue-ultraviolet avalanche photodiode till now. This invention gives such a blue-ultraviolet avalanche photodiode for the first time.
2. Description of Related Art
A CD (compact disc) and an MD (medium disc) have been widely employed as a recording media of music, movies or data. A CD player has a GaAs-type laser diode which emits near-infrared rays of a 780 nm long wavelength. Sales of the DVD players are going up at a rate of 300% a year. The present DVD players use another GaAs-type laser diode which emits red rays of 650 nm for reading-out data from DVD discs. Video-recorders which make use of rewritable DVD discs are on the market from the end of 1999. The shorter the wavelength of the light source of reading-out data from a DVD disc is, the drastically the data amount which can be stored in the DVD disc increases. Future progress of DVDs ardently desires still shorter wavelength light sources than the 650 nm GaAs-type lasers.
If a blue-violet laser diode of GaN (gallium nitride)-type is put on the market, the short wavelength light of 400 nm will be available for the light source of reading out data of DVDs.
In accordance with the appearance of 400 nm-emitting GaN-LDs on the market, commencement of sales of HD-DVDs which can record HD-TV for two hours is scheduled. Table 1 shows information capacity, recording hours, initially-manufactured years, colors of reading-out light source lasers, emission wavelengths of the lasers, materials of the lasers, kinds of photodetectors for the CD (compact disc), DVD (digital video disc), and HD-DVD (high definition digital video disc).
In the materials on the table, the listed xe2x80x9cGaAsxe2x80x9d means not the light emitting layer (active layer) but the material of the substrate. Another listed xe2x80x9cGaNxe2x80x9d is neither the material of the active layer nor the substrate but the material of typical films (buffer layer, contacting layer, or cladding layer). The xe2x80x9cGaNxe2x80x9d-type laser diode has a sapphire as a substrate and an InGaN layer as an active layer.
The 400 nm HD-DVDs which will be put on the market in 2002 can enhance the memory density by five times as much as the prior 650 nm-DVD. The HD-DVDs will have a 22.5 GB memory capacity which enables a DVD player to record HD-DVD movies for two hours.
An avalanche photodiode is a semiconductor photodetector having an avalanche amplification function which applies a strong reverse bias to a pn-junction, making a strong electric field in depletion layers and the pn-junction, accelerates photocarriers generated by light at the depletion layers, inducing reciprocal collisions of the accelerating photocarriers to lattice atoms, and generating new carriers by the collisions. The avalanche photodiode is an excellent photodiode having a built-in amplification function in itself.
A sensitivity range of a photodiode, in general, is determined by bandgaps of a material of a light receiving layer. A photodiode has a peak sensitivity at an absorption edge wavelength of the material of the light receiving layer. The photodiode has no sensitivity for the light having a longer wavelength than the absorption edge wavelength. The photodiode has sensitivity for the light having a smaller wavelength than the absorption edge wavelength. But, the sensitivity decreases as the wavelength becomes far shorter than the absorption edge wavelength. The tendency is common to the avalanche photodiode (APD). A Si-APD (visible) and a Ge-APD (infrared) are actually on the market. Various type InP-APDs have been proposed (for example, Japanese Patent Laying Open No. 60-198786(198786/""85) and Japanese Patent Laying Open No. 2-262379(262379/""90). But, the InP-APDs (infrared) are unstable and unreliable yet.
A Si-APD has sensitivity for visible light and near-infrared rays of wavelength between 500 nm and 900 nm. A Ge-APD has sensitivity for near-infrared rays. Thus, the Ge-APD and the Si-APD can cover the wavelength range from visible to near-infrared rays. An InP-APD with an InGaAs sensing layer can sense a wavelength range from 1200 nm to 1650 nm. However, the Ge-APDs and the InP-APDs are not put into actual use unlike the Si-APDs.
The Si-APD is made upon a p-type Si substrate unlike other silicon devices which are made on n-type Si substrates. The Si-APD is produced by piling a thin p-Si layer upon the p-Si substrate, making an n-region by thermally diffusing n-type dopant into the p-layer, and making an n-type Si guardring around the n-region by thermal diffusion for stabilizing electric field distribution at a periphery. The Si-device made on the p-type substrate is quite strange, since other Si devices are all made upon n-type Si substrates. Silicon is an ideal semiconductor in which a p-region can be made as easily as an n-region and holes have a similar mobility and an effective mass to electrons. Other semiconductor materials are asymmetric regarding a p-region and an n-region, and a hole mass and an electron mass, and a hole mobility and an electron mobility and so on.
Ge devices which are, in general, suffering from a loose pn-junction has large leak current and big dark current flowing across the pn-junction. An application of a large reverse bias induces a large increase of the dark current in a Ge-APD. Thus, the Ge-APDs and InP-APDs are not in practical use. The Si-APD is a unique practical avalanche photodiode still now.
The matter of the present invention is not a laser diode as a light source of irradiating DVDs for reading-in data but a pin-photodiode and an avalanche photodiode of sensing the reflected reading out light. Inexpensive GaN-type (InGaN) laser diodes will be manufactured at low cost on mass scale as light sources in near future. The progress of photodiodes for sensing the irradiated data with high quantum efficiency does not coincide with the progress of the laser diodes.
Silicon photodiodes (Si-PDs) have been used for sensing the reflected light data by light source lasers of CD players (780 nm) and DVD players (650 nm). The silicon photodiodes (Si-PDs) which have high sensitivity for visible light and near-infrared rays are optimum for sensing the 780 nm light (CDs) and the 650 nm light (DVDs). The silicon photodiodes, however, are incompetent for sensing 400 nm light (HD-DVDs).
FIG. 2 shows a graph of the quantum efficiency of pin-PDs as a function of wavelength. The abscissa is a wavelength (xcexcm) of light. The ordinate is theoretical optimized quantum efficiency (%). The graph shows the maximum efficiency for the diodes. All actual PDs have not such high sensitivity. Theoretical quantum efficiency of Si-PDs, Ge-PDs, InGaAs-PDs and AlGaAsSb-PDs are indicated. The most prevalent PDs for short wavelengths are Si-photodiodes. Silicon PD""s theoretical quantum efficiency which has a peak at 800 nm is 85% at 780 nm (CDs) and 70% at 650 nm (DVDs).
The sensitivity of Si-PDs falls down to 35% at 500 nm. The Si-PDs have poor sensitivity at 400 nm. The prevalent Si-PDs for the current CDs and DVDs are fully incompetent for sensing violet-blue rays of about 400 nm.
The new 400 nm light HD-DVDs require new photodetecting devices instead of Si-PDs. Phototubes and photomultipliers having vacuum tubes coated with SbCs, NaKSbCs, or GaAs(Cs) have sensitivity for near-ultraviolet rays between 300 nm and 400 nm. The phototubes and the photomultipliers which are not solid state semiconductor devices are unsuitable as photodetectors of domestic DVD players, because of large volume, heavy weight, complicated high-voltage power sources, short lifetime and high cost.
CdS (cadmium sulfide) is well known as a photoconductive material having sensitivity for visible light. A photoconductive device having no pn-junction has a drawback of slow response. The CdS photoconductive device cannot be a photodetector for reading out the HD-DVD. The HD-DVD requires a pin-photodiode or a pn-photodiode having a special sensing material which has sufficient sensitivity for the ultraviolet rays of the 400 nm wavelength.
One candidate for the HD-DVD reading-out device is an improvement of a matured visible silicon photodiode for extending the sensitivity range toward shorter wavelengths. Restriction of doping of impurity only near the surface may extend the sensitivity region toward blue rays.
The other candidate for the HD-DVD reading-out element is an entirely new photodetector employing a new material as a sensing layer. Since short wavelength light is an object, the material of the sensing layer should have a wide bandgap energy. In general, a pn-photodiode or a pin-photodiode has the maximum sensitivity for the light having the energy which is equal to the bandgap of the sensing layers of the photodiode.
The HD-DVD would employ GaN-type (i.e., InxGa1-xN/sapphire) LDs which produce blue, green or violet rays in accordance with mixture ratio x of In as a light source. Thus, a photodiode made of GaN may be the first candidate for the HD-DVD reading-out photodetector. A pair of a GaN-LD and a GaN-PD may be attractive for a pair of a light source and a detector for the HD-DVD. In the case, the GaN photodiode may be made upon a sapphire substrate like prevalent GaN-LDs, since a defectionless large single crystal GaN substrate cannot be obtained yet. A GaN/sapphire photodiode has been neither produced nor sold on the market. The reason is that GaN layers heteroepitaxially grown on a sapphire substrate has plenty of dislocations and other defects, and the defects induce large dark current and the defects decrease the sensitivity for a 400 nm wavelength.
Indeed, GaN/sapphire is an excellent blue LD. But, a GaN/sapphire would be a bad photodiode for blue-rays, even if the photodiode were to be fabricated.
The GaN/sapphire photodiodes are inoperative as a blue-violet photodiode for a 400 nm wavelength band due to the dark current and the low sensitivity.
One purpose of the present invention is to provide a pin-photodiode which can be used for reading out HD-DVD data by sensing light of a 405 nm wavelength.
Another purpose is to provide a pin-photodiode which senses blue-violet-near-ultraviolet rays has small dark current and is highly reliable as a photodetector.
There is no avalanche photodiode which senses blue-violet-near-ultraviolet rays of 460 nm to 300 nm yet. A further purpose of the present invention is to provide a blue-ultraviolet avalanche photodiode which can detect light of wavelengths from 300 nm to 460 nm.
Gallium nitride (GaN) and indium gallium nitride (InGaN) are very influential materials of blue-green light emitting diodes (LEDs) and laser diodes (LDs), because the bandgaps of GaN and InGaN correspond to blue-violet-ultraviolet. InGaN-LEDs have been already on the market and have been used in various fields. The prevailing InGaN-LEDs are all made upon a sapphire (Al2O3) substrate. However, InGaN/sapphire is not a suitable material for photodetectors of blue or violet rays. InGaN/sapphire has plenty of defects due to the misfit between the sapphire substrate and the InGaN films. An avalanche photodiode requires a strong electric field for inducing avalanche amplification. If an APD were made by the on-sapphire InGaN, large defect density of the on-sapphire InGaN would break the APD soon. Thus, InGaN is not a candidate for making violet-ultraviolet avalanche photodiodes.
The present invention proposes a Zn1-xMgxSySe1-y pin-photodiode having an n-type ZnSe single crystal substrate, an n-type Zn1-xMgxSySe1-y layer epitaxially grown on directly on the n-type ZnSe substrate or via an n-type ZnSe buffer layer on the n-type ZnSe substrate, an i-type Zn1-xMgxSySe1-y layer epitaxially grown upon the n-type Zn1-xMgxSySe1-y layer, a p-type Zn1-xMgxSySe1-y layer epitaxially grown on the i-type Zn1-xMgxSySe1-y layer, a p-type (ZnTe/ZnSe)m superlattice electrode epitaxially grown on the p-type Zn1-xMgxSySe1-y layer, a p-type ZnTe contact layer epitaxially grown upon the p-type superlattice electrode, a metallic p-type electrode formed upon the p-type ZnTe contact layer, and a metallic n-type electrode formed on the bottom of the n-type ZnSe substrate.
When the PD is a top incidence type, the metallic p-electrode should be either a small dot or an annular ring not to shield incidence light. Other parts on the top surface except the p-electrode should be coated with a protection film or an antireflection film.
A mixture crystal Zn1-x MgxSySe1-y has two parameters x and y. When x=0, Zn1-xMgxSySe1-y is reduced to ZnSySe1-y. When y=0, Zn1-xMgxSySe1-y is reduced to Zn1-xMgxSe. When x=0 and y=0, Zn1-xMgxSySe1-y is reduced to ZnSe. Thus, Zn1-xMgxSySe1-y is a collective concept including ZnSe, ZnMgSe, and ZnSSe.
The photodiode has a pin-structure consisting of a p-Zn1-xMgxSySe1-y layer, an i-Zn1-xMgxSySe1-y layer and an n-Zn1-xMgxSySe1-y layer. The i-Zn1-xMgxSySe1-y layer is a light sensing layer which absorbs light and makes pairs of holes and electrons. In the case of a top-incidence type PD, incidence light arrives via the upper p-layer at the i-layer. If the p-layer absorbs the light, photocurrent is reduced. Thus, absorption of the light in the p-layer reduces sensitivity of the PD. The p-layer absorption can be reduced by thinning the p-layer or enhancing a bandgap of the p-layer. An increment of x or y raises the bandgap of Zn1-xMgxSySe1-y.
Eg(p), Eg(i) and Eg(n) denote the bandgaps of the p-, i- and n-layers. There are two alternatives for the choice of materials for the p-, i- and n-layers.
One is an equivalent bandgap case. The other is a different bandgap case.
(1) An equivalent case (Eg(p)=Eg(i)=Eg(n))
In this case, all the p-, i-, and n-layers can be ZnSe. Otherwise, all the p-, i-, and n-layers can be ZnSySe1-y or Zn1-xMgxSe. Or all the layers can be Zn1-xMgxSySe1-y having the same x and y.
(2) A different bandgap case (Eg(p) greater than Eg(i)=Eg(n))
This case widens a bandgap of only the p-layer. The i-layer and the n-layer should have the same bandgaps smaller than the p-layer. The p-layer acts as a transparent window. The absorption loss at the p-layer is alleviated. One candidate set is a ZnSSe p-layer and ZnSe n- and i-layers. Another candidate is a ZnMgSSe p-layer and ZnSSe n- and i-layers. A further candidate is a ZnMgSSe p-layer with higher x and y and ZnMgSSe n- and i-layers with lower x and y.
Acquisition of sufficient sensitivity for near-ultraviolet (300 nm) rays requires the p-,i- and n-layers of Zn1-xMgxSySe1-y of x greater than 0.1 and y greater than 0.1. The Zn1-xMgxSySe1-y photodiode of x greater than 0.1 and y greater than 0.1 would have more than 50% quantum efficiency for 300 nm near-ultraviolet rays.
It is difficult for ZnSe-type photodiodes to form an ohmic contacting p-type electrode due to the wide bandgap of ZnSe. Wide bandgap materials suffer from a common difficulty of making an ohmic contact p-electrode. ZnTe which has a narrower bandgap than ZnSe is a unique material which can be easily converted into p-type conduction and can make an ohmic contact p-electrode.
ZnTe can be doped with As, P, Li or N as a p-type dopant in a wide range of dopant concentration. The p-type ZnTe can make an ohmic contact of a low resistance with a metallic p-electrode. Any 2-6 group compound semiconductors other than ZnTe do not have such a convenient property.
Thus, instead of p-ZnSe, p-ZnTe is chosen as a p-contacting layer with a p-metallic electrode. A p-ZnTe layer can make an ohmic p-contact with the metallic electrode. A choice of p-ZnTe as a p-electrode material induces a new problem. The top layer of the p-type layers is ZnTe. But, the intermediate p-layer is ZnSe, ZnSSe or ZnMgSSe which have wide bandgaps different from ZnTe. If ZnTe is directly piled upon the p-layer of ZnSe, ZnSSe or ZnMgSSe, the difference of the bandgaps will forbid current from flowing cross the boundary between ZnTe and ZnSe and so forth.
In order to conquer the discontinuity of the bandgaps, very thin p-ZnSe films and very thin p-ZnTe films are reciprocally piled in turn. The thickness of the p-ZnTe films is larger than p-ZnSe in the vicinity of the p-ZnTe top layer. The thickness of the p-ZnTe films should be decreased toward the p-ZnSe layer. This (ZnTe/ZnSe)m layered structure is called a superlattice electrode (SLE).
An antireflection film is a transparent film which suppresses reflection of incidence light at a surface for enhancing sensitivity of a photodiode. The reflection of an antireflection film has a wavelength dependence. One wavelength gives one antireflection film. There is no common antireflection film which would be applicable to all wavelengths in a wide wavelength range. The antireflection film fills another role of protecting a p-layer from degenerating.
Determination of one wavelength of object incidence light enables an operator to design an antireflection film suitable for the object wavelength. An antireflection film is made of a pile of transparent materials for the wavelength range, for example, Al2O3, SiO2, HfO2, TiO2, Ta2O5 or other dielectric materials. A sophisticated antireflection film can be produced by plenty of dielectric layers piled in series. A single dielectric layer having an effective quarter thickness (xcex/4n) of the wavelength can be a simple antireflection film.
Further, the present invention also proposes a Zn1-xMgxSySe1-y avalanche photodiode including an n-type ZnSe single crystal substrate, a low doped n-type Zn1-xMgxSySe1-y (0xe2x89xa6xxe2x89xa60.8;0xe2x89xa6yxe2x89xa60.8) avalanche layer epitaxially grown either directly upon the n-type ZnSe substrate or via an n-type ZnSe buffer layer piled upon the n-type ZnSe substrate, a p-type Zn1-xMgxSySe1-y (0xe2x89xa6xxe2x89xa60.8;0xe2x89xa6yxe2x89xa60.8) layer having a bandgap equal to or wider than a bandgap of the low doped n-type Zn1-xMgxSySe1-y layer, a p-type (ZnTe/ZnSe)m superlattice electrode which is made by piling p-type ZnTe thin films and p-type ZnSe thin films reciprocally, a p-type ZnTe contact layer epitaxially grown on the p-type superlattice electrode, a metallic p-type electrode formed upon the p-type ZnTe contact layer, a metallic n-type electrode formed on the bottom of the n-ZnSe substrate, and sides of all the epitaxially piled layers except the n-type ZnSe substrate being etched into a mesa, the etched sides being coated with an insulating film. The structure of p- and n-type metallic electrodes is the same as the before mentioned pin-PD electrodes.
The p-electrode on the top surface should be a small dot or a ring in order to introduce incidence light into the layered structure. The top surface except the p-electrode should be covered with a dielectric protection film or an antireflection film.
For simplicity, mixture ratios x and y are sometimes omitted in this description. ZnMgSSe is an abbreviation of Zn1-xMgxSySe1-y which includes x and y. When x=0, Zn1-xMgxSySe1-y is reduced to ZnSySe1-y. When x=0 and y=0,Zn1-xMgxSySe1-y is reduced to ZnSe. Thus, ZnMgSSe includes ZnSSe and ZnSe also.
This invention pairs the p-ZnMgSSe layer with the n-ZnMgSSe layer in order to detect violet-ultraviolet rays and makes a pn-junction between the p-ZnMgSSe layer and the n-ZnMgSSe layer. When a reverse bias is applied, depletion layers (i-layer) are bilaterally generated from the pn-junction to both the p-ZnMgSSe layer and the n-ZnMgSSe layer by the reverse bias. The depletion layer which has poor carriers is an intrinsic semiconductor. The depletion layer is sometimes called an xe2x80x9ci-layerxe2x80x9d. Thus, the structure is named a xe2x80x9cpinxe2x80x9d structure.
Incidence light arrives via the p-layer at the pn-junction. If a bandgap of the n-layer is equal to a bandgap of the p-layer, a part of the incidence light is absorbed in the upper p-layer before reaching the pn-junction. The absorption in the p-layer is a loss. Since the p-layer is thin, the absorption loss is small. When the absorption loss in the upper p-layer should be fully avoided, the p-layer should be composed of a material having a wider bandgap than the n-layer. The bandgap of the p-layer is denoted by Eg(p). The bandgap of the n-layer is denoted by Eg(n). There are two allowable cases.
(1) An equivalent case (Eg(p)=Eg (n))
A material of the p-layer is the same as a material of the n-layer. This equivalent case allows the p-layer to absorb a part of the incidence light. For example, the p- and n-layers are made of ZnSe. Otherwise, the p- and n-layers are composed of ZnSySe1-y of the same y. Or in general, the p- and the n-layers have a component of Zn1-xMgxSySe1-y with common ratios x and y.
(2) An inequivalent case (Eg(p) greater than Eg(n))
A material of the p-layer has a bigger bandgap energy than a material of the n-layer. The inequivalent case forbids the p-layer from absorbing the incidence light. The sensitivity is raised by avoiding the p-layer absorption loss. For example, the n-layer is ZnSe and the p-layer is ZnSSe or ZnMgSSe. Other example has an n-ZnSSe layer and a p-ZnMgSSe layer. Another example has a n-ZnMgSSe layer and a p-ZnMgSSe which has a bigger bandgap than the n-ZnMgSSe layer.
The present invention gives a ZnMgSSe pin-photodiode having an n-ZnSe substrate, an n-ZnMgSSe layer, an i-ZnMgSSe depletion layer, a p-ZnMgSSe layer, and a p-(ZnTe/ZnSe)m superlattice electrode for detecting blue-ultraviolet rays for the first time. An Si pin-photodiode which is suitable for visible and near-infrared rays has poor sensitivity for the blue-violet rays. A GaN/sapphire pin-photodiode has no sensitivity for blue-violet rays and poor sensitivity for ultraviolet rays. Beside the poor sensitivity, large dark current deprives the GaN/sapphire photodiode of practical value as a ultraviolet PD.
This invention gives a highly operative photodiode for blue-near-ultraviolet rays. The employment of the ZnSe substrate and the homoepitaxy of the pin-layers on the ZnSe substrate enable the ZnMgSSe-PD to enhance the sensitivity and the reliability by decreasing dark current and degradation.
The present invention also proposes a blue-ultraviolet avalanche photodiode with a long lifetime, high reliability and high sensitivity for the first time. The avalanche photodiode of the present invention has an n-ZnSe substrate, an n-ZnMgSSe mixture layer on the n-ZnSe substrate, a p-ZnMgSSe mixture layer on the n-ZnMgSSe layer, a p-(ZnTe/ZnSe)m superlattice electrode, and a p-ZnTe contact layer. A wide bandgap of ZnMgSSe enables the avalanche photodiode to sense blue, violet and near-ultraviolet rays. At present, available avalanche photodiodes are only a Si-APD and a Ge-APD. The Si-APD has sensitivity for visible light. The Ge-APD has sensitivity for infrared rays. There have never been a blue-ultraviolet sensitive APD. This invention proposes a blue-ultraviolet APD making use of ZnMgSSe with a wide bandgap. Unlike silicon, ZnMgSSe is suffering from a difficulty of making a p-type crystal, a difficulty of making a pn-junction, a difficulty of making a metallic p-electrode and a difficulty of making a good ZnSe single crystal substrate. Thus, the layered structure of the ZnMgSSe-APD is entirely different from the conventional Si-APD which has a p-Si substrate, pn-junctions made by thermally diffusing n-dopants or p-dopants, and guardrings. Furthermore, the upper sides of the APD chip of the present invention are etched and are coated with protecting, insulating layers. The ends of the pn-junction are protected by the protecting layers, which ensures a long lifetime.
A breakdown voltage of the APD depends upon the dopant concentration of the n-layer and the p-layer. Embodiment 4 gives a xe2x88x9227 V breakdown voltage. A xe2x88x9225 V reverse bias amplifies the gain fifty-times as strong as a 0 V bias.
There is only a photomultiplier as a photodetector which has high sensitivity for short wavelength (blue-violet-ultraviolet) light. The photomultiplier is a bulky, heavy, expensive, ill-operative, fragile photodetector which requires a heavy, large power source and a light shield for prohibiting external noise light from breaking a phototube.
The present invention is the first light, inexpensive, good-operative, sturdy blue-ultraviolet sensitive semiconductor photodetector. Gallium nitride (GaN) seems to be a strong candidate for ultraviolet photodetectors due to a wide bandgap. However, since GaN films are heteroepitaxially made on a sapphire substrate, the GaN films are suffering from plenty of defects which will be rapidly grown and will break a pn-junction by application of strong reverse bias. GaN is unsuitable for a material of avalanche photodiodes. The ZnMgSSe-APD is an epochmaking, unique APD operative for the short wavelength range of blue, violet and near-ultraviolet rays.